Wrenches for turning fasteners such as nuts and bolts are available in a variety of configurations, including fixed jaws, adjustable jaws, or sockets. Open-end wrenches have fixed jaws at one or both ends of a handle which define a U-shaped wrench cavity for accepting the head of a fastener. These wrenches are primarily designed for use with hexagonal fastener heads but also may be used with any fastener head having an equal number of faces. To turn the fastener, the wrench is placed over the fastener with the flat jaws of the wrench aligned with flat surfaces on the head of the fastener, and force is exerted on the handle.
One problem experienced with conventional open-end wrenches is that the jaws often do not remain aligned with the flat surfaces when force is applied to the handle but rotate relative to the flat surfaces and contact the comers of the fastener head, thereby marring or rounding off the comers of the fastener. This relative rotation results in part from manufacturing tolerances inherent in the manufacture of wrenches and variances in the head sizes of the fasteners among manufacturers. Because of these variations, open-end wrenches are generally ineffective for turning fastener heads that are not hexagonal in shape or that have worn or rounded-off corners, especially if great force is required to turn the fastener.
Many attempts have been made to design an open-end wrench that reduces marring or rounding off of the corners of a fastener head and that can be used with marred fastener heads and non-hexagonal (particularly square) fastener heads. One example of an open-end wrench designed for this purpose is shown in U.S. Pat. No. 2,685,219 to Diebold. The wrench cavity shown by Diebold includes two curved bosses for contacting opposite flat surfaces of a fastener head. While providing some improvement over conventional open-end wrenches, this design suffers certain disadvantages in that it is not usable with fasteners having square heads due to its geometry. Likewise, the shape of the cavity tends to force a fastener away from the wrench when a large amount of torque is applied because of the position of its lower bosses.
Another drawback of Diebold is that the cavity exerts force primarily at the comers of a fastener head. For example, using the Diebold disclosure, a minimum free swing angle (in this case 21.0 degrees), median wrench and 3/4 inch hex fastener tolerances, calculations show that Diebold engages the side of a fastener at a distance of 0.04 inches from the corner. (The median wrench opening tolerance is 0.759 inches and the median fastener tolerance for 3/4 inch hex fastener is 0.743 inch. See ANSI B107.6.) The closer the point of contact on which force is exerted is to a comer of a fastener, the greater the chance for marring.
Another example of an open-end wrench designed to reduce rounding-off of the fastener comers is shown in U.S. Pat. No. 3,242,775 to Hinkle. While this design is intended to apply force to the flat faces or surfaces of a nut instead of the corners (Col. 2, lines 15-18), in actuality the disclosed wrench applies force at or close to the comers of the fastener (see FIGS. 8 & 12). For example, calculations made using the Hinkle disclosure, a minimum free-swing angle (in this case 3.5 degrees) and median wrench and fastener tolerances, show that Hinkle engages the side of a fastener at a distance of 0.07 inches from the comer.
It is also significant that Hinkle specifically teaches away from the use of a curved surface as the drive surface by claiming that such actually increases the indentation of the fastener. See col. 9, lines 71-75 and col. 10, lines 1-8. Use of such curved contact surfaces to reduce marring, however, is known in the industry. Examples of curved contact surfaces include U.S. Pat. No. 4,930,378 to Colvin, though this patent only discloses closed wrench applications. Even if one were to assume an open wrench design, however, calculations done using the disclosure in Colvin, a minimum free-swing angle (in this case 4.0 degrees) and median wrench and fastener tolerances show that Colvin would engage a fastener only 0.09 inches from a comer.
Colvin is also limited in the amount of torque it can apply to a fastener due to the presence of intersecting flat and arched surfaces that tend to create stress risers in that the point of intersection tends to take most of the load. While others have attempted to eliminate this stress riser problem by using continuously curved surfaces, for example U.S. Pat. No. 4,581,957 to Dossier, no improvement has been made in contacting the side of a fastener closer to the center. For example, using the Dossier disclosure, a median free-swing angle (in this case 4.0 degrees) and median wrench opening tolerances and fastener tolerances, and assuming an open wrench application, calculations still show a maximum point of contact at 0.09 inches from the comer of a fastener.
Therefore, despite the various efforts found in the prior art, there remains a need for an improved open-end wrench which can be used to turn a variety of fastener head configurations without marring.